In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels and the like, the tissue sheet is formed by depositing an aqueous suspension of papermaking fibers onto a forming fabric. The web is then transferred to a papermaking felt and dewatered as it passes through a pressure nip created between a pressure roll and a Yankee dryer as the wet web is transferred to the Yankee surface. Free water expressed from the web in the pressure nip is absorbed and carried away by the felt as the web transfers to the Yankee surface. The web is then final dried on the surface of the Yankee and subsequently creped to impart bulk and softness to the resulting tissue sheet. This method of making tissue sheets is commonly referred to as xe2x80x9cwet-pressingxe2x80x9d because of the method used to dewater the wet web.
The wet-pressing method has a couple of distinct drawbacks. First, pressing the tissue web while wet densifies the web significantly. As the web is dried, the dried sheet retains this high density (low bulk) until it is creped. Creping is necessary to attempt to undo what the wet-pressing has done to the sheet. In response to this situation, through-air-drying methods have been developed in which the newly-formed web is partially dewatered to about 30 percent consistency using vacuum suction. Thereafter the partially dewatered web is final dried without compression by passing hot air through the web while it is supported by a throughdrying fabric. However, through-air-drying is expensive in terms of capital and energy costs.
A second drawback, shared by conventional wet-pressing and through-air-drying processes is the high energy costs necessary to dry the web from a consistency of about 35 percent to a final dryness of about 95 percent. This second drawback has recently been addressed in the manufacture of high density paper products by the advent of the high intensity extended nip press. This device employs an extended nip length and heat to more efficiently dewater the wet web up to exit consistencies of about 60 percent. Such devices have been successfully used for making paperboard, but have not been used to make low density paper products such as tissues because the high pressures and longer dwell times in the extended nip press serve to further densify the sheet beyond that experienced by conventional tissue wet-pressing methods. This increase in density is detrimental to the quality of the resulting tissue products because creping cannot completely overcome the added increase in sheet density.
Therefore there is a need for a method of making wet-pressed tissue sheets that minimizes or eliminates the high densities imparted to wet-pressed tissue webs.
It has now been discovered that the reduction in bulk associated with wet-pressing can be substantially reduced by incorporating into the web certain fibers which have been found to greatly diminish web densification when subjected to the high pressures necessary for dewatering with high intensity extended nip presses. As a consequence, high intensity extended nip presses can be used to dewater tissue webs without the heretofore adverse consequence of imparting a high degree of densification to the web.
Hence in one aspect the invention resides in a method for making a bulky tissue sheet comprising: (a) depositing an aqueous suspension of papermaking fibers onto a forming fabric to form a wet tissue web, said papermaking fibers comprising at least about 10 dry weight percent modified wet-resilient fibers; (b) partially dewatering the wet web to a consistency of about 15 percent or greater; (c) compressing the partially dewatered web in a high intensity extended nip press to further dewater the web to a consistency of about 35 percent or greater; and (d) final drying the web, wherein the Bulk of the dewatered web prior to final drying is greater than (xe2x88x920.02C+3.11), wherein xe2x80x9cCxe2x80x9d is the consistency of the web leaving the high intensity extended nip press, expressed as percent dryness, and Bulk is expressed as cubic centimeters per gram. For a given consistency, the wet tissue webs of this invention have greater bulk than comparable wet tissue webs that have been dewatered by conventional means. Furthermore, the consistency can be increased well beyond that attainable by conventional tissue dewatering and, in most instances, still have a higher bulk at higher consistencies than that of conventional wet tissue webs at substantially lower consistencies.
In another aspect, the invention resides in the combination of dewatering a tissue web using a high intensity extended nip press, which greatly reduces the bulk of the tissue web, followed by rush transferring the dewatered web to increase the bulk of the web back to levels suitable for tissue. More specifically, the invention resides in a method for making a bulky tissue sheet comprising: (a) depositing an aqueous suspension of papermaking fibers onto a forming fabric to form a wet tissue web; (b) partially dewatering the wet web to a consistency of about 15 percent or greater; (c) compressing the partially dewatered web in a high intensity extended nip press to further dewater the wet web to a consistency of about 35 percent or greater; (d) transfering the dewatered web to a first transfer fabric; (e) transfering the dewatered web from the first transfer fabric to a second transfer fabric travelling at a slower speed than the first transfer fabric (rush transfer) to increase the bulk of the wet web; and (f) drying the web. The web can be dried on a Yankee dryer and creped, or the web can be throughdried and left uncreped or creped.
As used herein, xe2x80x9cmodified wet-resilient fibersxe2x80x9d are fibers that have been modified from their natural state and have the capability to recover after deformation in the wet state, as opposed to fibers that remain deformed and do not recover after deformation in the wet state. Examples of modified wet-resilient fibers include, without limitation, chemically cross-linked cellulosic fibers, heat-cured cellulosic fibers, mercerized fibers and sulfonated pulp fibers. These fiber modification methods are well known in the art. The amount of modified wet-resilient fibers in the fiber furnish can be about 10 dry weight percent or greater, more specifically from about 20 to about 80 percent, and still more specifically from about 30 to about 60 percent. The bulk benefits associated with using modified wet-resilient fibers increase as the amount of the modified wet-resilient fibers increases. Consequently the amount used must take into account the desireability for added bulk versus other desired properties, such as tensile strength, that other fibers may be better suited to provide.
A xe2x80x9chigh intensity extended nip pressxe2x80x9d, as used herein, is a water-removing pressing apparatus wherein the wet web is compressed in an extended nip formed between the arcuate surface of a backing roll and a pressing fabric or blanket. Typically the pressing fabric is supported by a press shoe having a concave surface. The backing roll can be heated to elevated temperatures or remain at ambient temperature. The length of the extended nip can be substantial, typically from about 5 to about 10 inches or more. Such devices permit the operator to vary conditions such as dwell time, pressure and temperature to effect greater water removal than can normally be obtained in a conventional roll press. Such an apparatus can remove substantially all of the free water in the sheet and a significant portion of the bound water as well. An example of such an apparatus is disclosed and described in U.S. Pat. No. 4,973,384 issued Nov. 27, 1990 to Crouse et al. entitled xe2x80x9cHeated Extended Nip Press Apparatusxe2x80x9d, which is herein incorporated by reference. In operating the high intensity extended nip press, the use of a heated press roll in the extended nip is optional, although preferred for maximum water removal.
The consistency (weight percent fiber or percent dryness) of the partially dewatered web entering the high intensity extended nip press can be about 15 percent or greater, more specifically from about 15 to about 30 percent. The consistency of the web leaving the high intensity extended nip press can be about 35 percent or greater, more specifically from about 40 to about 70 percent, and still more specifically from about 50 to about 65 percent. The final consistency may depend upon the incoming web consistency, the speed of the web, the temperature of the heated roll, the pressure within the nip, the length of the nip, the properties of the fibers and the characteristics of the press felt, as well as additional variables.
Depending upon the consistency to which the web is dewatered and other factors, such as the temperature/pressure of the high intensity extended nip press and the dwell time in the nip, the Bulk of the wet web leaving the high intensity extended nip press can be from about 2.3 to about 3.5 cubic centimeters per gram or greater, more specifically from about 2.4 to about 3.0 cubic centimeters per gram. More specifically, taking the consistency of the web into account, the Bulk of the wet web leaving the high intensity extended nip press can be greater than (xe2x88x920.02C+3.11), more specifically greater than (xe2x88x920.032C+3.78), still more specifically greater than (xe2x88x920.02C+3.52), and still more specifically greater than (xe2x88x920.03C+4.28), where xe2x80x9cCxe2x80x9d is the consistency of the web. The origin of these values will be described in detail in reference to the Drawings. Stated differently, the increase in Bulk attained when using the high intensity extended nip press to dewater webs containing modified wet-resilient fibers is from about 5 to about 50 percent, more specifically from about 10 to about 40 percent, and still more specifically from about 20 to about 30 percent greater than the Bulk of webs consisting of a 50/50 weight percent blend of eucalyptus and northern hardwood kraft fibers produced under the same conditions.
As used herein, Bulk is determined by dividing the caliper of the web by the basis weight. The caliper is measured for a single web or sheet using a T.M.I. Model 549 micrometer (Testing Machines Inc., Amityvile, N.Y.) using a circular pressure foot having an area of 200 square millimeters. The pressure foot lowering speed is about 0.8 millimeters per second. The pressure, when lowered, is about 0.50 kilogram per square centimeter. The dwell time is about 3 seconds. One measurement is taken for each sheet and five sheets of each sample are tested. The readings are taken near the end of the dwell time for each test. The average of the five readings is the caliper of the sample.
In those embodiments of this invention in which a rush transfer is utilized after the web has been dewatered, the speed of the first transfer fabric (the fabric from which the web is being transferred) can be about 5 to about 35 percent faster than the speed of the second transfer fabric (the fabric to which the web is being transferred). More specifically, the speed differential can be from about 10 to about 30 percent, and still more specifically from about 20 to about 30 percent. As the speed differential is increased, the Bulk of the resulting web is increased. Speed differentials greater than about 35 percent, however, are not desirable because the sheet buckles to form macrofolds. dr
FIG. 1 is a schematic diagram of a tissue making process in accordance with this invention, illustrating the use of a high intensity extended nip press.
FIG. 2 is a schematic view of the high intensity extended nip press, illustrating its function in more detail.
FIG. 3 is a plot of Bulk as a function of web consistency for handsheets produced under conditions simulating the operation of a high intensity extended nip press, illustrating the decrease in Bulk with increasing exit consistency for a number of different fiber furnishes.